Safety valves in tubular strings such as a production string in a borehole or a production riser from a subsea wellhead at times need to be held open. Early designs managed to lock the valve open in such a way that further functionality of the valve was destroyed. One example is U.S. Pat. Nos. 7,137,452; 7,703,541 and U.S. Pat. No. 5,598,864. Other designs used a flow through method to open the flapper and combined flow through the passage in cycles of pressure actuation and removal with a j-slot mechanism to hold a flapper open and another cycle to release the flapper for normal operation. These designs left the flapper open to the flow path where accumulated debris could impede the movement of the hold down mechanism or the flapper. Some examples of this are U.S. Pat. Nos. 7,527,104 and 8,607,811. Some devices would disable the safety valve and obtain access to the hydraulic system to run other tools. An intervention into the string was required to do this. In some applications like marine risers there is a 90 degree bend in the riser near the platform preventing inserting tools to lock open the valve while also disabling its hydraulic system from resuming normal operation. One example is U.S. Pat. No. 7,717,185. Another design involved delivering and expanding a sleeve to hold the flapper open and disable the safety valve from further normal operation and is shown in U.S. Pat. No. 6,684,958. U.S. Pat. No. 7,779,919 shows the use of a primary piston to manipulate a flow tube during normal operation and a second hydraulic piston not operably connected to the flow tube that could retain the flapper open. When hydraulic pressure was removed a spring bias allowed release of the flapper to resume normal operation with the flow tube. This design left the flapper exposed to debris in well fluid when locked open. In another design the back of the flapper could selectively engage a hook latch in the open position after being pushed down by the flow tube. A cable release could either prevent the flapper from latching when shifted to open or allow retaining the flapper with a hook entering a recess in the back of the flapper until the flow tube was raised clear of the flapper. A cable could then remove the hook from the back of the flapper allowing it to swing closed for normal operation when the flow tube was then raised up. This design is shown in US 2007/0137869 and it does not appear to be intended to function as a lock open device but rather in high flow situations to avoid flapper or flow tube damage from high flow closing the flapper against a flow tube that is not retracted fast enough by a closure spring.
U.S. Pat. No. 9,422,790 illustrates a flow tube operated flapper where a ratchet can hold the flow tube in the extended position so that the valve is locked open. A tool can then be inserted into the flow tube to latch into the flow tube.
U.S. Pat. No. 9,394,762 shows a debris barrier movable against a flow tube to keep well fluid debris away from the flapper in the open position when the flapper is behind the flow tube.
Probably the most relevant reference with regard to the present invention is U.S. Pat. No. 5,167,284 which shows a main piston associated with a flow tube for normal operation of the flapper for the open and closed positions. A secondary piston moves a one way ratchet through a selectively releasable retainer. The ratchet assembly holds the flow tube in the down position effectively locking the valve open. Release occurs by applying the pressure on the main piston and relieving pressure on the secondary piston which allows a plate 80 to be pushed down to spread the outer ratchet. Bleeding pressure off of the primary and secondary pistons allows the locking secondary piston to full retract so that the flow tube can move up and normal operation of the safety valve can resume. This complex design has a ratchet exposed to well fluids that can get stuck and fail to release the locking piston from trying to push plate 80 down with the primary piston. If the ratchet fails to sufficiently retract the valve stays locked open. The secondary piston cannot operate the valve at all and further features an array of small parts and springs calling into question its reliability in severe environments.
What is needed and provided by the present invention is a flow tube operated flapper that has redundant capability for moving the flow tube when using a primary or a secondary piston. The secondary piston is linked with an indexing feature to respond to pressure cycles to selectively lock the flow tube in the flapper open position or with another pressure cycle on the secondary piston to release the flow tube for normal operation with the primary piston. The flapper can be held open with pressure on the secondary piston in a configuration that if the control pressure on the secondary piston is lost the closure spring will shift the flow tube for a fail-safe configuration of the flapper to the closed position. The flow tube in the locked open position can be a clearance fit to the surrounding housing to minimize debris infiltration to the volume where the open flapper resides behind the flow tube. These and other aspects of the present invention will be more readily apparent to those skilled in the art from a review of the description of the preferred embodiment and the associated drawings while recognizing that the full scope of the invention is to be determined from the appended claims.